Among the decliners, shares of Biogen Idec
BIIB, +4.02%
shed $1.17, or 2.1 percent, to $55.70. Shares of both Biogen Idec and partner Elan soared last week after the companies said they would apply for approval of a new multiple sclerosis therapy in mid-2004, a year ahead of schedule. See full story.

The companies said on Wednesday that they would apply for approval of the MS drug, called Antegren, based on one-year results of clinical testing that's scheduled to run for two years. Although Elan and Biogen Idec didn't disclose the clinical data, investors interpreted the news positively and surmised the one-year results are already strong enough to support drug approval.

But some analysts have sounded a note of caution on Antegren. Analyst Bill Tanner at Leerink Swann & Co. said many questions remain about the still-experimental therapy. Tanner said Antegren could also hurt sales of Avonex, Biogen Idec's top-selling multiple sclerosis therapy.

"Antegren could have a drastic effect on the MS drug marketplace, Avonex included," Tanner wrote in a note to clients.

Among the day's leading advancers, shares of Vertex Pharmaceuticals
VRTX, +1.97%
tacked on 76 cents, or 7.4 percent, to $10.97. Vertex said one of its experimental cancer-fighting compounds showed significant promise in testing on mice. However, the company would need to successfully complete extensive testing in patients before it could even apply for regulatory clearance to market the drug. See full story.

In the pharmaceutical sector, shares of Merck & Co.
MRK, +1.80%
a component of the Dow Jones Industrial Average, edged up 23 cents to $48.54. S.G. Cowen said in a research note it expects the Food and Drug Administration to decide within days whether to accept for review Merck's application to market a new arthritis medication, Arcoxia.

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